Are you referring to an RV air conditioner with a "heat strip" option?  That
is the only "combination" unit I am aware of.

To my knowledge, the heat strip is good for little more than "taking the chill
off" a cool morning.

As for efficiency:  Electric is nearly 100% efficient.  Watts is watts,
regardless of the model.

No.  That is assuming you are referring to a "heat strip" optioned AC unit.

Below freezing there is no substitute for a good gas furnace except for one or
two good electric space heaters - perhaps three.

A few years ago I read about some smart guy that was putting home ac
generator heads under the hoods of automobiles, run by the car engine.
These would be stand-by home power outage ac generators, run by a belt on a
pulley, which would be mechanically engaged only when needed.  The car would
essentially run at idle (since most home generators use roughly 10 hp).  It
would be cheaper than a standard generator because you don't need a small
gas engine - only the generator head, pulleys., etc.

This sounds like a great idea for remote pop-up camping.  However, now I
can't find anything info on the web about this.

Has anybody else heard of this?